The present invention relates generally to holographic devices, and more particularly, to a switchable hologram.
Switchable holograms are often formed from polymer-dispersed liquid crystal (PDLC) material which have holographic fringes recorded therein. During the recording process, the PDLC material undergoes a phase separation thereby creating fringes, which include regions densely populated by liquid crystal micro-droplets interspersed with regions of clear polymer. The liquid crystal material is interposed between electrodes so that an electrical field can be applied to the hologram. When an electrical field is applied to the hologram, the natural orientation of the liquid crystal droplets is changed, causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to a very low level, thereby effectively erasing the hologram.
Volume holograms (also known as thick or Bragg holograms) provide high diffraction efficiencies for incident light beams having wavelengths close to the theoretical wavelength satisfying the Bragg diffraction condition, and which are within a few degrees of the theoretical angle that also satisfies the Bragg diffraction condition. Experimental evidence shows that the magnitude of the electric field required to switch these holograms between their active (diffracting) state and passive (non-diffracting) state is dependent upon the slant angle of the holographic fringes, which is defined as the angle between the Bragg surface of the fringes and a normal to the hologram surface.
In applications in which a hologram is required to operate over a wide range of incidence angles, such as with an eyepiece in an off-axis wearable display device, there is likely to be significant variation in the slant angle over the area of the hologram. When an electric field of a predetermined magnitude is applied to the hologram, there may be areas in which the slant angle prevents the hologram from switching (i.e., the switching threshold due to the slant angle is higher than the applied field). Under these circumstances, the hologram will only be partially erased.
The magnitude of the electric field may be increased to overcome this problem. However, there may be other areas within the hologram which are then exposed to excessively high voltages (i.e., areas where the switching threshold due to the slant angle is significantly below the applied voltage). This may cause electrical breakdown, delamination (i.e., separation of the hologram from the substrate), or other effects that result in irreversible damage to the hologram.